Work vehicles for agricultural use need the capacity both to pull large loads and the ability to turn in a small radius. Agricultural tractors require high load-pulling capacity since they often tow heavy implements over rough terrain such as plows, harrows, fertilizer carts and others. At the same time, they require a tight turning radius since the implements towed are typically used for cultivating fields, and thus must make tight turns at the end of the field to permit the cultivation of adjacent rows.
To provide for ever increasing load-pulling requirements, tractor manufacturers have historically increased tire diameters (for wheeled vehicles) or the length of track contacting the ground (for tracked vehicles). Increasing wheel diameter or track length is limited in many current tractor designs, however. Increasing diameter or length in tractors that employ wheel end or wagon-type steering only will cause the wheel or track to hit the sides of the vehicle when turned, thus requiring the allowable steering angle to be reduced. To avoid this problem, tractor manufacturers previously moved to articulated vehicles that have a front and rear frame and are steered by pivoting about an articulation joint in the middle of the vehicle.
The use of purely articulated vehicles solved the problem temporarily, but articulated vehicles had additional problems that appeared when wheel and track size increased to provide additional load-pulling capacity. Since the hinge or pivot points for articulated tractors were typically disposed in the center of the vehicle, enlarged wheels and tracks caused the trailing surfaces of the front wheel or track and the leading surfaces of the rear wheel or track to touch when the vehicles were steered.
To prevent this interference yet maintain a small turning radius, tractor manufacturers developed vehicles having a combination of articulation steering and wheel-end steering. The degree of articulation was reduced from the previous generation to prevent wheel-to-wheel interference, thus increasing the effective turning radius. The turning radius was reduced, however, by reintroducing wheel-end steering. By combining an articulation joint with a reduced steering angle and a wheel-end steering to compensate, increased wheel and track sizes could be provided while keeping the same turning radius. Another approach was to provide a vehicle with two articulation joints disposed longitudinally along the centerline of the vehicle, thereby splitting the vehicle into three longitudinal sections.
The solution that combined wheel-end and articulated steering and the solution using double articulated steering have been outgrown, and proposed new designs again face the problems of interference between the front and rear wheels and tracks. To provide more load-pulling power, vehicles with wider tracks or a pair of large wheels on each side of the tractor ("duals") both front and rear have been proposed. These vehicles, due to their increased width, larger tracks or larger wheels again present the problem of wheel-to-wheel or track-to-track interference in tight turns. What is needed therefore is a new steering arrangement for agricultural tractors that will permit increased wheel sizes, vehicle width and track width yet not adversely affect turning radius. It is an object of the present invention to provide such an arrangement.